More Acidic Oceans Could Reduce Fertility for Algae Eaters

New research shows that increased levels of carbon dioxide in the oceans cause changes that alter key nutrients essential to the reproduction of animals low on the food web.

Copepods provide the link between algae and larger organisms in many marine food webs. New research shows that when female copepods eat algae grown under conditions that simulate ocean acidification, they lay fewer eggs (blue in the photo above), and their eggs are less viable. Credit: Matt Wilson/Jay Clark, NOAA NMFS AFSC, CC BY 2.0

As the world’s oceans absorb increasing amounts of carbon dioxide from the atmosphere, their waters become more acidic. In addition to known harm from increased acidity to the skeletal growth of marine organisms such as corals, this shift in ocean chemistry could lead to fertility problems for animals at the base of the marine food web, new experiments suggest.

Studies dating back more than a decade have shown that increased acidity changes the composition of fatty acids in marine microalgae. Now, researchers report that when tiny crustaceans called copepods ate a diet of microalgae with those altered fatty acids, the algae eaters produced fewer eggs, and fewer of their offspring survived.

Because copepods and the algae they consume support nearly all marine ecosystems, the results suggest that ocean acidification could have harmful consequences that might ripple throughout the food web, said Morgan Meyers, a marine biologist at the Romberg Tiburon Center for Environmental Studies in Tiburon, Calif., who led the research. The Tiburon center is part of San Francisco State University, in San Francisco, Calif.

The study “gives us a broader picture of how ocean acidification affects the food web beyond a single species.”“Although it is difficult to know how these conditions would affect higher trophic levels, this gives us a broader picture of how ocean acidification affects the food web beyond a single species,” said Meyers on Thursday, when she presented her team’s findings at the 2016 Ocean Sciences Meeting in New Orleans, La.

Acidity Limits Essential Nutrients

Algae are the ocean’s primary producer of polyunsaturated fatty acids and provide their predators with an important source of essential fatty acids—fats that animals cannot make for themselves but are needed for proper growth and reproduction.

To test how ocean acidification could affect algae and their consumers through fatty acids, Meyers and her colleagues grew four types of microalgae in seawater containing levels of dissolved carbon dioxide corresponding to current and future ocean acidity levels. The scientists then fed a mixture of the four types of microalgae to copepods caught in San Francisco Bay. They measured how many eggs the copepods produced, how many eggs hatched into larvae, and how many of those larvae survived.

The team confirmed that increased acidity reduces the amount of essential fatty acids that microalgae produce, but the effect is different for each species: Some types of algae are more susceptible to environmental changes than others. When the researchers then fed the microalgae to copepods, they found that changes in algal fatty acid composition due to acidification significantly affected the copepods’ reproductive success.

The copepods that ate microalgae grown under future acidity conditions produced 47% fewer eggs than those that ate microalgae grown under present conditions.The copepods that ate microalgae grown under future acidity conditions produced 47% fewer eggs than those that ate microalgae grown under present conditions. In addition, fewer of their eggs hatched: 36% compared to 92% for copepods that ate algae grown under current conditions. Of the eggs that did hatch, only 13% of larvae survived, compared to 97% of the larvae from water in present conditions.

Previous studies have shown that copepods that ate microalgae under similar conditions produced fewer eggs, but the study by Meyers and her team is the first to assess the viability of these eggs.

Effects on the Food Web

The effects of environmental changes on algal fatty acid composition have been well documented, but it is unclear whether these changes are biologically significant, said Aaron Galloway, a marine ecologist at the Oregon Institute of Marine Biology in Coos Bay, who was not connected to the research. “What’s important about Meyers’s work is that she took that next step: By feeding algae that were cultured under these conditions to copepods, she’s showing that the differences in algal fatty acids may not only be statistically significant but also potentially biologically relevant” to other organisms in marine food webs, he said.

Because the copepods ate a mixture of algal species that responded in different ways to increased acidity, it’s difficult to tease apart the ecological significance of the results, according to Galloway. “A good follow-up to this study would be to see what happens when copepods are forced to eat just one of these species of algae,” he said.

According to Galloway, Meyers’s study shows that the effects of ocean acidification can be transferred from producer to consumer, but it is unclear whether scientists will see these effects at the next trophic level and higher up in the food web because higher consumers draw from more prey sources.

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